Theoretical insights into nucleation of CO2 and CH4 hydrates for CO2 capture and storage

Abstract

We present a hybrid three-dimensional (3D) theoretical approach, the density functional theory (DFT) integrated with the reference interaction site model (RISM), to investigate the nucleation of CO₂ and CH₄ hydrates. Within the theoretical framework, the 3D-RISM is applied to describe gas density distributions in hydrate cages, and the 3D-DFT is used to describe the interfacial structure and properties of the two hydrates, as well as their nucleation. The crystal–liquid phase equilibria of CO₂ and CH₄ hydrates are predicted by the hybrid 3D-DFT-RISM, and compared with the available experimental data to examine the theoretical model. In particular, the local and interfacial structure and properties, the critical nucleus radii and free-energy barriers at moderate concentration supersaturation are presented to analyze their nucleation. The formation enthalpies for the two hydrates are calculated to evaluate the possibility of CO₂ storage by CH₄–CO₂ replacement in hydrate.